The present invention relates to a method for leakage testing in a tank system and a device for carrying out such a method.
The requirements for emission control for motor vehicles constantly increase. In connection with such requirements, there are also requirements for detecting that there are no emissions due to leaks in the tank system of the vehicle. If a leak is detected, this is usually indicated to the driver by a control system in the vehicle switching on a light on the instrument panel of the vehicle, a so-called MIL (Malfunction Indication Lamp). An example of a requirement regarding detection of leaks in the tank system of a vehicle is the CARB OBDII Leakage Detection Requirement (California Air Resources Board, On Board Detection) which requires the detection of leaks in a tank system which have a flow which corresponds to the flow through a circular hole, the diameter of which exceeds a certain given limit, which is currently 1 mm.
One method of detecting leaks in the tank system of a vehicle is to seal the tank system during a measuring period, and to create an overpressure or an underpressure in the tank system. If the pressure in the sealed tank system varies more than a certain permitted value during the measuring period, this is taken as an indication of a leak in the tank system. An example of this method is, for example, that shown in U.S. Pat. No. 5,261,379, which uses an underpressure.
A drawback of the method of detecting leaks using pressure variations in a sealed system is that pressure variations can have other causes than leaks. Transient occurrences such as, for example, fuel sloshing can cause pressure increases since sloshing leads to an increased evaporation, and thus to an increased pressure.
This means that during a leakage measurement there can be other circumstances, among them fuel sloshing, due to which the results of the leakage test should not be accepted, since the results of the test are not reliable.
A method or system for leakage testing which cannot recognize circumstances which can cause erroneous measurements can function well in a controlled environment, such as in a test lab where it is possible to check that such conditions are not present. However, under real circumstances, such as under normal driving conditions, such a system will cause erroneous detections of leaks in the tank system.
In conclusion, there is a need to be able to tell when there are circumstances in a tank system, for example fuel sloshing, the presence of which mean that the result of the leakage test should not be accepted.
The object of the present invention is thus to obtain a method which can recognize circumstances, the presence of which means that the result of a leakage test in a tank system should not be accepted.
In accordance with the present invention, these and other objects have now been realized by the invention of a method for testing leakage in a tank system comprising sealing the tank system, creating a pressure variation in the sealed tank system, measuring the pressure values in the sealed tank system at predetermined time intervals for a predetermined measuring period, and comparing the shape of a first curve formed by the measured pressure values to a predetermined second curve, whereby if the comparison exceeds a predetermined limit value the leakage test is disregarded. In a preferred embodiment, the step of creating the pressure variation in the sealed tank system comprises creating an underpressure in the sealed tank system. In another embodiment, however, the step of creating the pressure variation in the sealed tank system comprises creating an overpressure in the sealed tank system.
In accordance with one embodiment of the method of the present invention, comparing of the shape of the first curve to the predetermined second curve comprises fitting a function with the predetermined second curve to the shape of the first curve using the method of least squares, calculating, squaring and summing the differences between the function of the predetermined second curve and the measured pressure values of the first curve, dividing the sum by the number of the measured pressure values to provide a predetermined ratio, calculating a curving 2a2 value for the predetermined second curve, and comparing the predetermined second curve to the first curve using the predetermined ratio and the curving 2a2 value. In accordance with another embodiment of the method of the present invention, the predetermined second curve comprises a second degree polynomial.
In accordance with another embodiment of the method of the present invention, the predetermined time intervals are constant. In accordance with one embodiment of the method of the present invention, the predetermined limit value is based upon a condition comprising fuel sloshing.
In accordance with another embodiment of the method of the present invention, the number of the measured pressure values comprises an odd number.
In accordance with present invention, an apparatus has also been devised for testing leakage in a tank system comprising sealing means for sealing the tank system, pressure variation means for exposing the sealed tank system to a predetermined pressure variation, pressure measuring means for measuring the pressure in the sealed tank system, and memory means adapted to store predetermined pressure values for the tank system and for comparing the shape of a first curve formed by the measured pressure values to a predetermined second curve, whereby if the comparison exceeds a predetermined limit value the leakage test is disregarded.
In accordance with one embodiment of the apparatus of the present invention, the pressure variation means comprises means for exposing the sealed tank system to an underpressure.
In accordance with another embodiment of the apparatus of the present invention, the pressure variation means comprises means for exposing the sealed tank system to an overpressure.
According to a preferred embodiment of the present invention, the tank system is first sealed, and subsequently the pressure in the tank system is lowered. Following this, a number of values of the pressure in the tank system are then measured during a measuring period. The curve which is formed by the measured pressure values is then compared to the shape of a pressure variation curve which is known to form during circumstances which allow leakage testing. If the discrepancy between the measured curve and the curve with which it is compared exceeds a certain predefined limit, a circumstance is indicated which means that the result of the leakage testing in the tank system should not be accepted.